NAME

     vsamp - resample a  velocity  worktape  in  preparation  for
     prestack depth migration algorithm sammig.

SYNOPSIS

     vsamp [ -Nfile_vsm ] [ -Ofile_vsamp ]  [  -Zfile_irreg  ]  [
     -Tfile_twt ] [ -dxvdxv ] [ -dzvdzv ] [ -xvoxvo ] [ -zvozvo ]
     [ -zmaxzmax ] [ -freffref ] [ -vsrvsr ] [  -dzeqldzeql  ]  [
     -FlDtEl ] [ -V ] [ -? ]

DESCRIPTION

vsamp is designed to read in a smoothed velocity worktape
and resample it at irregular depths. This irregular depth
grid is defined by the input velocity file and a user speci-
fied number of points per wavelength at a given reference
frequency. The resampling is done in such a manner that
vertical traveltimes are preserved.

The input regularly gridded velocity worktape is assumed to
represent the velocity field averaged over an area (-
dxgrid/2:+dxgrid/2) (-dzgrid/2:+dzgrid/2) centered about
each grid point. Interpolation for resampling is done in a
manner that preserves vertical two way travel times in the
following manner:

1) if the first trace is at grid location ixgrid=1, the
input velocity worktape represents an average of the velo-
city field at the grid point:
(x=xvo+(ixgrid-1)*dxgrid,z=zvo+(izgrid-1)*dzgrid).

2) calculate the vertical two way travel time
twtgrid(izgrid,ix)
at each depth point (xvo+(ixgrid-1)*dxgrid,zvo+(izgrid-
1)*dzgrid)).
The two way travel time is integrated from a floating
datum, zdatum, defined by the trace header word FlDtEl if
the -FlDtEl option is given. If -FlDtEl is not specified,
the two way travel time is integrated from the top of the
model, zmin.

3) calculate an irregular grid zeta(izeta) which samples the
depth field using vsr points per wavelength.

4) calculate a 2nd irregular grid zetamid(izeta) that falls
half way between the nodes of zeta(izeta).

5) calculate the vertical travel time
twt(zetamid(izeta),ixgrid) by linearly interpolating the
travel times from the regular grid values,
twt(zeta(izeta),ixgrid).

6) calculate an effective velocity v(izeta,ix) centered
about the depth point ((ixgrid-1)*dxgrid,zeta(izeta)) using
the formula:

vsamp(izeta)=(zetamid(izeta+1)-zetamid(izeta))/(twt(zetamid(izeta+1)-twt(zetamid(izeta)).

Both the irregularly sampled output velocity worktape and
the ascii file (following the -Z option) containing the
irregular depths will be used by routines kray and tiray,
2D Kirchhoff depth migration ray trace algorithms. Breaking
this module out of the MBS algorithm itself serves two pur-
poses: the interpreter is able to QC his/her depth sampling
and input velocity worktape, and the algorithm developer is
able to decouple the details of model smoothing and ray
tracing from the migration algorithm proper.

vsamp gets both its data and its parameters from command
line arguments. These arguments specify the input, outputs,
and certain floating point values to override the current
restrictions on seismic trace header integer values.

Command line arguments
-N file_vsm
Enter the input velocity file name immediately after
typing -N unless the input is from a pipe in which case
the -N entry must be omitted. This input file should
include the complete path name if the file resides in a
different directory. Example
-N/export/data1/white_river/vsm tells the program to
look for file 'vsm' in directory
'/export/data1/white_river'.

-O file_vsamp
Enter the output resampled velocity data set name or
file immediately after typing -O unless output is to a
pipe in which case the -O entry must be omitted. The
output data set also requires the full path name (see
above). The first sample and trace of the output velo-
city worktape correspond to the first sample and trace
of the input velocity worktape, that is, to the argu-
ments following the -xvo and -zvo keywords.

-Z file_irreg
Enter the output depth sample file name immediately
after typing -Z. The file following the -Z option will
be the input file following the -Z option in routines
kray, tiray, sammig and zstretch. This output data set
requires the full path name (see above).

-T file_twt
Enter the output two way travel time file name immedi-
ately after typing -T. The two way traveltime
(measuered in ms) is calculated for each input velocity
trace. This file will be used in stretching the
migrated data set (crps, common offset or stacked sec-
tions) to time in program tstretch. This output data
set requires the full path name (see above).

-dxv dxv
Enter the spatial increment between traces on the
seismic velocity worktape. Default is calculated by
multiplying trace header word DX1000*.001 where DX1000
has been filled in by one of the MBS velocity model
building programs.

-dzv dzv
Enter the depth increment between samples on the
seismic velocity worktape. Def ault is calculated by
multiplying trace header word DZ1000*.001 where DZ1000
has been filled in by one of the MBS velocity model
building programs.

-xvo xvo
Enter the x position of the first trace of the seismic
velocity worktape in m (ft). For prestack depth migra-
tion, this coordinate must be consistent with that of
the RecInd of the seismic data fed into routine prep-
mig. For example, if the first trace of the velocity
model corresponds to seismic station location (RecInd)
500 and the distance between station locations (dsta)
is 26.6667m, then xvo = 13333.35m. To force the user to
ponder the model/seismic data relationship, there is no
default. xvo will be saved in lineheader word MnTrOf.

-zvo zvo
Enter the z position of the first sample of the seismic
velocity worktape in m (ft). This will be the elevation
of the top of the velocity model. For prestack depth
migration, this coordinate must be be greater than the
seismic source and receiver elevations (header words
GrpElv and SrPtEl) extracted from routine prepmig. Folk
traditions require that zvo be positive for a value
lying above sea level. zvo will be saved in lineheader
word MxRSEL. As above, no default.

-zmax zmax
Enter the maximum depth (below sea level) to be resam-
pled, and to be imaged by subsequent migration program
sammig. Velocities generated below the extent of the
input velocity model will be assigned a value equal to
that at the bottom of the input model. Careful folks
will observe that 'up' is positive when defining topog-
raphy with the -zvo option, 'down' is positive when
defining maximum depth with the -zmax option. Very
good. Don't be stressed out, routine vsamp will define
a new variable zmin=-zvo, to make things consistent. As
above, no default.

-fref fref
Enter the reference frequency at which the -vsr (verti-
cal samling rate) option is specified. Defaults to 50
Hz.

-vsr vsr
Enter the vertical sampling rate per wavelength at
reference frequency, fref, to resample the velocity
worktape. Since we are dealing with 2 way propagation,
the default is twice the Nyquist of one way propaga-
tion, that is, 4 points per wavelength.

-dzeql dzeql
Enter the constant resampling depth increment. If this
value is present and positive , an equally sampled
depth grid will result. It this value is negative, an
equally sampled grid will be generated with dzeql set
to be the depth increment in the input file trace
headers. If dzeql=0 or the option is not invoked an
irregularly sampled depth grid and velocity model as
defined by the -fref and -vsr option will be generated.
Default value is 0., i.e. go generate an irregularly
sampled grid.

-maxzeta maxzeta
Enter the maximum number of irregular depth points
allowed in the output velocity model (Default:
maxzeta=2000).

-FlDtEl
Enter the command line argument -FlDtEl to set two
way travel time outputin file file_twt to be zero at
the floating datum detph, zdatum(ixgrid), correspond-
ing to the trace header word FlDtEl.

-V Enter the command line argument '-V' to obtain verbose
output.

-? Enter the command line argument '-?' to get online
help. The program terminates after the help screen is
printed.

NOTES

     NOTE 1:  In order to stage data for P/SV migration the  user
     will need to invoke the constant reference grid (-dz) option
     such that subsequent rayfans willbe traced onto a consistent
     grid.

BUGS

     No bugs known at present.

AUTHOR

     K. J. Marfurt (APR,  1992)   extracted  from  S.  H.  Gray's
     algorithm  smallmig  /  smigA. Interpolation to preserve two
     way travel time preservation suggested by  N.  D.  Whitmore.
     (Updated, August, 1996 by K. J. Marfurt, Amoco EPTG)

COPYRIGHT

     copyright 2001, Amoco Production Company
               All Rights Reserved
          an affiliate of BP America Inc.

Man(1) output converted with man2html